1. Field of the Invention
The present invention relates to an image processing technique of converting the frame rate of a moving image.
2. Description of the Related Art
CRTs have long been used as moving image display devices typified by a television receiver. However, panels using liquid crystal devices are becoming mainstream recently. When the liquid crystal device displays a moving image whose frame rate is 60 Hz, each pixel maintains emission for 1/60 sec. The liquid crystal device is therefore called a “hold type” device.
Further, field-emission-type display devices having the same emission characteristics as those of CRTs are currently under development. When a display device of this type displays a moving image whose frame rate is 60 Hz, light is emitted only instantaneously in 1/60 sec. Hence, the display device of this type is called an “impulse type” device.
The hold-type device easily generates a motion blur. The impulse-type device suffers a prominent flicker. Japanese Patent Laid-Open No. 2006-184896 discloses a technique to reduce a motion blur upon display on the hold-type device by raising the frame rate of a moving image. More specifically, two types of subframe images, that is, a subframe image containing only low-frequency components and one in which high-frequency components are enhanced are generated from one input frame image of a moving image whose frame rate is 60 Hz. These subframe images are alternately output as an output frame image in one cycle in 120 Hz. The original frame image is reproduced in one cycle in 60 Hz by alternately displaying the subframe image containing only low-frequency components and one in which high-frequency components are enhanced. The motion blur is reduced in a moving image whose frame rate is increased by this method, compared to an original moving image whose frame rate is 60 Hz.
However, when the method disclosed in Japanese Patent Laid-Open No. 2006-184896 is used, an image obtained by compositing the two types of frame images (that is, an image the viewer views) may become different from the original frame image. That is, the output image does not look the same as the original one. The viewer may perceive this as degradation.
The method disclosed in Japanese Patent Laid-Open No. 2006-184896 will be explained with reference to FIG. 4. A waveform 1901 exemplifies the waveform of an input frame image. A waveform 1902 is obtained by enhancing the low-frequency components of the waveform 1901 of the input frame image. A waveform 1903 represents high-frequency components obtained as the difference between the waveform 1901 (input frame image) and the waveform 1902 (low-frequency components). In the method disclosed in Japanese Patent Laid-Open No. 2006-184896, the high-frequency component waveform 1903 is added to the waveform 1901, generating a waveform 1904. In one cycle in 120 Hz, the waveforms 1902 and 1903 are alternately displayed. Theoretically, an apparent waveform becomes identical to the waveform 1901 by alternately displaying the waveforms 1902 and 1903 in one cycle in 120 Hz.
However, when the waveform 1901 has a value of 0 or almost 0, the waveform 1904 sometimes takes a negative value. Since an image having a negative value cannot be displayed, the negative value is displayed as “0” in practice, like a waveform 1905. In this case, the waveforms 1902 and 1905 are alternately displayed, so the apparent waveform becomes a waveform 1906. When a white character is displayed on a black background, the viewer perceives a blurred outline of the character.